CHAPTER 12 MEASUREMENTS WITH THE MINI-PAM
12.3 Assessment of photosynthesis with the MINI-PAM: Outline
of the most important functions in practical applications.
As soon as the MINI-PAM is switched on, it continuously
monitors the fluorescence yield of a sample which is close to the
fiberoptics exit. In section 12.1 it was outlined, in which way
fluorescence yield relates to the effective quantum yield of
photochemical energy conversion. Assessment of this very
fundamental information is made automatically by two consecutive
measurements of fluorescence yield (initiated by START), one
briefly before and one during a short pulse of saturating light. The
effective quantum yield of photochemical energy conversion (Y,
YIELD) is then simply calculated from the equation Y = ∆F/Fm.
Although this sounds easy and straightforward, in practice certain
aspects must be taken into consideration to obtain optimal and
meaningful results (for a brief outline, see section 5). While it is
almost trivial that the actual measurement must be correct, it is also
important that the conditions are properly chosen to give meaningful
information. Both of these two aspects are dealt with in the following
sections, which outline the most important functions of the MINI-
PAM, corresponding to some selected points of the MODE-menu. A
short description of all 51 points of the MODE-menu is given in
section 7.
12.3.1 Maximal photochemical yield Fv/Fm
In green plants the maximal quantum yield of photosystem II is
observed after dark adaptation when all reaction centers are open (all
primary acceptors oxidized) and heat dissipation is minimal. Then a
saturation pulse induces maximal fluorescence yield, Fm, and
maximal variable fluorescence, Fv, such that also ∆F/Fm = Fv/Fm is
maximal. Fv/Fm, if properly assessed, is a reliable measure of the
potential quantum yield of PS II. It is lowered by all effects which
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